Interviewer: x !== x can be true ?

Author: fatfish
Translator: Front-end Xiaozhi Source: medium

If you have dreams and dry goods, you can search for [Great Move to the World] on WeChat and pay attention to this Shuwanzhi who is still washing dishes in the early hours of the morning.

This article GitHub https://github.com/qq449245884/xiaozhi has been included, there are complete test sites, materials and my series of articles for interviews with first-line manufacturers.

foreword

Recently, I was asked a few weird interview questions. They are different from normal questions: these interview questions seem very simple, but they test your thorough understanding of JavaScript. How many can you answer correctly?

x !== x can be true ?

Excuse x will be printed 大迁世界

 const x = ? // ？？
if (x !== x) {
  console.log('大迁世界')
}

Strange, is there any value that is not equal to itself? There is indeed a value in JS NaN which is not equal to any value, not even itself.

 const x = NaN 
if (x !== x) {
  console.log('大迁世界')
}
console.log(NaN === NaN) // false
console.log(x !== x) // true
console.log(Number.isNaN(x)) // true

(!isNaN(x) && x !== x) can be true?

Now that we rule out NaNs, what other value can be not equal to itself?

 const x = ? 
if(!isNaN(x) && x !== x) {
  console.log('hello fatfish')
}
view rawq2-1.js hosted with

Maybe you know object. Defineproperty which can help us solve this problem.

 window.x = 0 
Object.defineProperty(window, 'x', {
  get () {
    return Math.random()
  }
})
console.log(x) // 0.12259077808826002
console.log(x === x) // false
console.log(x !== x) // true

3. How to make x === x + 1

This question may not be easy, but as long as you know JS, you will know Number.MAX_SAFE_INTEGER , which means that the constant represents the largest safe integer in JavaScript ( 2^53 - 1 ).

So, we can assign Number.MAX_SAFE_INTEGER to x :

 const x =  Number.MAX_SAFE_INTEGER + 1 
if (x === x + 1) {
  console.log('大迁世界')
}

4. x > x can be true ?

I don't want to read any more, what rubbish question is this?

 const x = ? 
if (x > x) {
  console.log('hello fatfish')
}

Although it seems unlikely, how can a value be greater than itself? However, we can use the Symbol.toPrimitive function to do this.

 const x = {
  value: 1,
  [ Symbol.toPrimitive ] () {
    console.log('x', this.value)
    return --this.value
  }
}

if (x > x) {
  console.log('大迁世界')
}

Really amazing (garbage) odd (garbage).

5. typeof x === 'undefined' && x.length > 0

 const x = ? 
if(typeof x === 'undefined' && x.length > 0) {
  console.log('大迁世界')
}

I have to admit, JS is an amazing language. Besides undefined itself, what value could make typeof x === undefined to be true ?

The answer is document.all , which represents all elements on the page.

 const x = document.all 
if(typeof x === 'undefined' && x.length > 0) {
  console.log('大迁世界')
}

console.log(x)
console.log(typeof x)
console.log(x === undefined)

At last

What strange interview questions have you encountered, please leave a message to add! !

---

The bugs that may exist in editing cannot be known in real time. In order to solve these bugs afterwards, a lot of time is spent on log debugging. By the way, here is a useful BUG monitoring tool , Fundebug .

Original: https://javascript.plnenglish.io/interviewer-can-xx-return-true-in-javascript-7e1d1a7b5cd

comminicate

If you have dreams and dry goods, you can search for [Great Move to the World] on WeChat and pay attention to this Shawanzhi who is still washing dishes in the early hours of the morning.

This article GitHub https://github.com/qq449245884/xiaozhi has been included, there are complete test sites, materials and my series of articles for interviews with first-line manufacturers.